The present invention relates to a plastic cup incorporating a pilfer-proof function.
As is disclosed in U.S. Pat. No. 4,550,884, for example, conventional plastic-molded pilfer-proof caps have a cap skirt and a band member which are integrated by means of a plurality of frangible bridges. More than 10 stopper tabs are formed on an internal wall surface of the band member by way of projecting themselves in the obliquely downward direction. When screwing the cap skirt on the bottle mouth, the stopper tabs come into contact with a bead member extending outwardly in the periphery of the bottle mouth, and then reversely bend in the obliquely upward direction. Then, while maintaining the bent condition, the stopper tabs climb over the bead member before eventually being engaged with the bottom surface thereof to prevent the band member from being lifted while opening the bottle. Each of the stopper tabs is thin at the bent end. However, the nearer the projected end, the greater the thickness of each stopper tab.
Japanese Patent Publication No. HEI3-11985 (1991) discloses a pilferproof cap. This prior art discloses a plurality of stopper tabs on an internal wall surface of a band member, and the stopper tabs extend in the obliquely upward direction. Like the above example, the nearer the bend end, each of these stopper tabs has a thinner surface wall. This is because of the need to minimize transit resistance while the stopper tabs climb over the bead member. Excessive transit resistance causes bridges to easily be torn off while screwing the cap on the bottle mouth.
The applicant for a patent related to the present invention previously proposed a pilfer-proof cap via Japanese Laid-Open Patent Application Publication No. HEI1-182259 (1989), which provides a plurality of stopper tabs projecting from the bottom edge of a band member in the obliquely upward direction, wherein a surface wall of each stopper tab is bent in a chevron shape in a sectional view.
Each of the stopper tabs has a surface wall that is thin at the refracted end and thicker in the direction of projected end, and the transit resistance of these stopper tabs can be minimized when climbing over the bead member on a bottle mouth. On the other hand, there is a problem in terms of the retentive force of the band member while opening the bottle cap. Accordingly, while the band member is pulled in the upward direction relative to a cap opening operation, the refracted ends and projected tips of the stopper tabs respectively bend downwardly. As a result, the band member shifts upward by such an amount corresponding to the degree of deformation incurred. In an extreme case, the band member is fully disengaged from the secured position.
If the stopper tabs were incapable of sustaining enough engaging force, even though the sealed bottles were opened out of mischief while being displayed, nobody could visually identify the earlier opening of the bottles. When the band member shifts upwardly, the seal of the cap leaves the bottle mouth before the bridges are torn off, thus resulting in the faulty effect of sealing. If this symptom occurs in a bottle containing a carbonated beverage, carbonic acid gas will be lost. Unless the bridges are torn off, dissipation of carbonic acid gas cannot be identified.
In order to provide stopper tabs with a greater engaging force, for example, by increasing the thickness of the bent ends of the stopper tabs, the change would result in an increased transit resistance while the stopper tabs climb over the bead member as the cap is screwed on the bottle mouth. If excessive force were applied in order to screw the cap on the bottle mouth, the bridges could be torn off.
As is apparent from the above, it is a matter of contradiction to minimize the transit resistance while the stopper tabs respectively climb over the bead member while capping a bottle and simultaneously provide the stopper tabs with greater engaging force while opening the bottle.
On the other hand, according to the cap having the surface walls of the stopper tabs that are folded in the chevron shape, the surface walls of the stopper tabs are provided with greater buckling strength than that of conventional plane-shaped stopper tabs, thus permitting the chevron-shaped stopper tabs to more securely retain the band member. However, like the above-cited conventional cap, the previously proposed cap is also provided with a number of independent stopper tabs on the internal surface of the band member. In consequence, these stopper tabs bend themselves inwardly or downwardly while opening a bottle thereby causing the seal to be released before the bridges are torn off. In other words, there was a certain limit in improving the strength of the stopper tabs to resist deformation during the opening of a bottle.